Laguna, from Final Fantasy VIII, was planned to be included in the game as head of the Mirage Arena but Square Enix was also planning for him to be in Dissida 012 Final Fantasy which was being developed around the same time. Developer Nomura Takahashi decided that they wanted Laguna to be in Dissidia more and made the decision to leave him out of Birth by Sleep.
Contributed by Shinitenshi